This "scary and invasive capability" has been included in almost every larger piece of software for years and is widely accepted to be a mostly good thing - it's called automatic updates. Considering updates allow pushing native, even admin-level code, this capability of pushing little bits of JS becomes benign in comparison. Therefore, the only thing that's left to worry about are their intentions. And I, for one, would rather trust the goodness of Mozilla's intentions than Google's or Microsoft's.